Although bioactive peptides can be produced chemically by a variety of synthesis strategies, recombinant production of peptides, including those in the 5-50 amino acid size range, offers the potential for large scale production at reasonable cost. However, expression of very short polypeptide chains can sometimes be problematic in microbial systems, including in bacterial cells such as Escherichia coli. This is true even when the peptide sequence is expressed as part of a fusion protein. As part of a fusion protein, peptides may be directed to specific cellular compartments, i.e. cytoplasm, periplasm, or media, with the goal of achieving high expression yield and avoiding cellular degradative processes.
Preparation of a peptide from a fusion protein in pure form requires that the peptide be released and recovered from the fusion protein by some mechanism and then obtained by isolation or purification. Methods for cleaving fusion proteins have been identified. Each method recognizes a chemical or enzymatic cleavage site that links the carrier protein to the desired protein or peptide [Forsberg et al., I. J. Protein Chem. 11, 201-211, (1992)]. Chemical cleavage reagents in general recognize single or paired amino acid residues which may occur at multiple sites along the primary sequence, and therefore may be of limited utility for release of large peptides or protein domains which contain multiple internal recognition sites. However, recognition sites for chemical cleavage can be useful at the junction of short peptides and carrier proteins. Chemical cleavage reagents include cyanogen bromide, which cleaves at methionine residues [Piers et al., Gene, 134, 7, (1993)], N-chloro succinimide [Forsberg et al., Biofactors 2, 105-112, (1989)] or BNPS-skatole [Knott et al., Eur. J. Biochem. 174, 405-410, (1988); Dykes et al., Eur. J Biochem. 174, 411-416, (1988)] which cleaves at tryptophan residues, dilute acid which cleaves aspartyl-prolyl bonds [Gram et al., Bio/Technology 12, 1017-1023, (1994); Marcus, Int. J. Peptide Protein Res., 25, 542-546, (1985)], and hydroxylamine which cleaves asparagine-glycine bonds at pH 9.0 [Moks et al., Bio/Technology 5, 379-382, (1987)].
Of interest is U.S. Pat. No. 5,851,802 which describes a series of recombinant peptide expression vectors that encode peptide sequences derived from bactericidal/permeability-increasing protein (BPI) linked via amino acid cleavage site sequences as fusions to carrier protein sequences. In some fusion protein constructs, an acid labile aspartyl-prolyl bond was positioned at the junction between the peptide and carrier protein sequences. BPI-derived peptides were released from the fusion proteins by dilute acid treatment of isolated inclusion bodies without prior solubilization of the inclusion bodies. The released peptides were soluble in the aqueous acidic environment. In addition, BPI-derived peptides were obtained from fusion proteins under conditions where the fusion proteins were secreted into the culture medium. Those secreted fusion proteins were then purified and treated with dilute acid to release the peptide.
Of additional interest are the disclosures of the following references which relate to recombinant fusion proteins and peptides.
Shen, Proc. Nat'l. Acad. Sci. (USA), 281, 4627 (1984) describes bacterial expression as insoluble inclusion bodies of a fusion protein encoding pro-insulin and .beta.-galactosidase; the inclusion bodies were first isolated and then solubilized with formic acid prior to cleavage with cyanogen bromide.
Kempe et al., Gene, 39, 239 (1985) describes expression as insoluble inclusion bodies in E. coli of a fusion protein encoding multiple units of neuropeptide substance P and .beta.-galactosidase; the inclusion bodies were first isolated and then solubilized with formic acid prior to cleavage with cyanogen bromide.
Lennick et al., Gene, 61, 103 (1987) describes expression as insoluble inclusion bodies in E. coli of a fusion protein encoding multiple units (8) of .alpha.-human atrial natriuretic peptide; the inclusion bodies were first isolated and then solubilized with urea prior to endoproteinase cleavage.
Dykes et al., Eur. J. Biochem., 174, 411 (1988) describes soluble intracellular expression in E. coli of a fusion protein encoding .alpha.-human atrial natriuretic peptide and chloramphenicol acetyltransferase; the fusion protein was proteolytically cleaved or chemically cleaved with 2-(2-nitrophenylsulphenyl)-methyl-3'-bromoindolenine to release peptide.
Ray et al., Bio/Technology, 11, 64 (1993) describes soluble intracellular expression in E. coli of a fusion protein encoding salmon calcitonin and glutathione-S-transferase; the fusion protein was cleaved with cyanogen bromide.
Schellenberger et al., Int. J. Peptide Protein Res., 41, 326 (1993) describes expression as insoluble inclusion bodies of a fusion protein encoding a substance P peptide (11a.a) and .beta.-galactosidase; the inclusion bodies were first isolated and then treated with chymotrypsin to cleave the fusion protein.
Hancock et al., WO94/04688 (PCT/CA93/00342) and Piers et al. (Hancock), Gene, 134, 7 (1993) describe (a) expression as insoluble inclusion bodies in E. coli of a fusion protein encoding a defensin peptide designated human neutrophil peptide 1 (HNP-1) or a hybrid cecropin/mellitin (CEME) peptide and glutathione-5-transferase (GST); the inclusion bodies were first isolated and then: (i) extracted with 3% octyl-polyoxyethylene prior to urea solubilization and prior to factor X.sub.a protease for HNP1-GST fusion protein or (ii) solubilized with formic acid prior to cyanogen bromide cleavage for CEME-GST fusion protein; (b) expression in the extracellular supernatant of S. aureus of a fusion protein encoding CEME peptide and protein A; (c) proteolytic degradation of certain fusion proteins with some fusion protein purified; and (d) proteolytic degradation of other fusion proteins and inability to recover and purify the fusion protein.
Lai et al., U.S. Pat. No. 5,206,154 and Callaway, Lai et al. Antimicrob. Agents & Chemo., 37:1614 (1993) describe expression as insoluble inclusion bodies of a fusion protein encoding a cecropin peptide and the protein encoded by the 5'-end of the L-ribulokinase gene; the inclusion bodies were first isolated and then solubilized with formic acid prior to cleavage with cyanogen bromide.
Gramm et al., Bio/Technology, 12:1017 (1994) describes expression as insoluble inclusion bodies in E. coli of a fusion protein encoding a human parathyroid hormone peptide and a bacteriophage T4-encoded gp55 protein; the inclusion bodies were first isolated (6% wt/vol.) and then were treated with acid to hydrolyze the Asp-Pro cleavage site.
Kuliopulos et al., J. Am. Chem. Soc., 116:4599 (1994) describes expression as insoluble inclusion bodies in E. coli of a fusion protein encoding multiple units of a yeast .alpha.-mating type peptide and a bacterial ketosteroid isomerase protein; the inclusion bodies were first isolated and then solubilized with guanidine prior to cyanogen bromide cleavage.
Pilon et al., Biotechnol. Prog., 13, 374-379 (1997) describe soluble intracellular expression in E. coli of a fusion protein encoding a peptide and ubiquitin; the fusion protein was cleaved with a ubiquitin specific protease, UCH-L3.
Haught et al., Biotechnol. Bioengineer., 57, 55-61 (1998) describe expression as insoluble inclusion bodies in E. coli of a fusion protein encoding an antimicrobial peptide designated P2 and bovine prochymosin; the inclusion bodies were first isolated and then solubilized with formic acid prior to cleavage with cyanogen bromide.
The above-references indicate that production of small peptides from bacteria has been problematic for a variety of reasons. Proteolysis of some peptides has been particularly problematic, even where the peptide is made as a part of a larger fusion protein. Such fusion proteins comprising a carrier protein/peptide may not be expressed by bacterial host cells or may be expressed but cleaved by bacterial proteases. In particular, difficulties in expressing cationic antimicrobial peptides in bacteria have been described by Hancock et al. WO94/04688 (PCT/CA93/00342) referenced above, due in their view to the susceptibility of such polycationic peptides to bacterial protease degradation.
The production of peptide for preclinical and clinical evaluation often requires multigram quantities [Kelley, Bio/Technology 14, 28-31 (1996)]. If production of recombinant peptides can be achieved at this large scale, such production can potentially be economical. However, downstream processing steps for the production of peptides and proteins from bacteria can often contribute a significant fraction of total production cost. Initial recovery of peptide from bacterial inclusion bodies of fusion proteins, for example, generally requires multiple distinct processing steps, including the following four steps: (1) cell disruption/lysis, (2) isolation of inclusion bodies from the disrupted/lysed cells, (3) solubilization of the isolated inclusion bodies in denaturant or detergent to obtain solubilized fusion protein, and (4) fusion protein cleavage and separation of peptide and carrier protein. It is desirable that aspects of the recombinant production process be improved and/or optimized in order to make large-scale production of peptides by recombinant means more economically viable.
There continues to exist a need in the art for improved methods for recombinant production of peptides from bacterial cells, particularly for simpler methods that do not require a multiplicity of steps, including, for example, the step of isolation or purification of peptide fusion proteins or the step of isolation or purification of inclusion bodies comprising the fusion proteins in order to obtain the recombinant peptide.